The present invention relates to methods and apparatuses for data compression.
A header is a part of the transmitted data which contains information used for handling the data transmission, in contrast to the payload which represents the actual data to be transmitted.
Headers are for example used in cell-based or packet based data transmission, wherein each data cell or each data packet has its own header. General examples for such headers are headers of IP packets or header of ATM cells (Asynchronous Transfer Mode). It should be noted that in general no clear delimitation between the terms “packet” and “cell” exists, since both terms relate to entities having a header and payload data. However, usually the term packet is used in a different layer in a layered communication model, e.g. the OSI model (Open Systems Interconnection) than the term cell. In the following, only the term cell will be used to generally designate a portion of data to be sent together with its header and is intended to also cover data packets fulfilling this requirement. An example for such cells used in the embodiments which will be described in the following are ATM cells, ATM being a conventional mode for transmitting data, for example for transmitting data from a central office to customer's premises, for example a home or an office, for DSL (Digital Subscriber Line) communication.
In order to keep the overhead generated by the headers of the cells, i.e. the additional header data to be transmitted, as small as possible, header compression algorithms are used. For example, for ATM header compression in the transmission convergence layer (ATM-TC header compression), a 5-byte ATM header may be compressed to a 1-byte header using a lookup table.
In some cases, headers of cells comprise a numbering information designating a consecutive numbering of the cells. An example for this in case of ATM transmission is IMA+ bonding according to the ITU (International Telecommunication Union) recommendation ITU-T G.998.1. IMA (Inverse Multiplexing over ATM) bonding is a technique wherein the ATM cells are transmitted over a plurality of parallel links, for example separate wires, in order to increase the bandwidth. In case of said IMA+ bonding, a bonding layer specific sequence index (SID) is included in the headers of the ATM cells in the form of an 8-bit or 12-bit field incremented by one for each following ATM cell which will be passed from a bonding entity or layer to an ATM-TC entity or layer. This SID is an example for the numbering information mentioned above and may, in case of IMA+ bonding, be used to reconstruct the correct order of the cells received over the plurality of parallel links mentioned above.